It is commonplace in the electronic art to combine a modulated signal with a local oscillator signal in order to obtain a further modulated signal at another frequency that is more easily amplified and detected. This is done in a mixer.
In a typical application, a modulated radio frequency (RF) signal is combined in a mixer with a local oscillator (LO) signal to produce an intermediate frequency (IF) signal which may be then further amplified and detected to recover the information modulated onto the RF carrier. The mixing process produces sum and differences of the RF and LO frequencies. One or more of the sum and difference frequencies is at the desired IF frequency, according to the following relations: EQU f.sub.IF =f.sub.LO -f.sub.RF, i.e., down-conversion where f.sub.LO &gt;f.sub.RF, (1) EQU f.sub.IF =f.sub.RF -f.sub.LO, i.e., down conversion where f.sub.LO &lt;f.sub.RF, (2) EQU f.sub.IF =f.sub.LO +f.sub.RF, i.e. up conversion. (3)
Examination of equations (1) and (2) shows that there is not a unique correspondence between f.sub.LO, f.sub.IF and f.sub.RF. For a given value of f.sub.LO, two different values of f.sub.RF may produce the same value of f.sub.IF. For example, (see FIG. 1) for f.sub.LO =3 GigaHertz, both f.sub.RF 1=2.5 GigaHertz and f.sub.RF 2=3.5 GigaHertz can produce f.sub.IF =0.5 GigaHertz. The RF and IF frequencies are generally not discrete frequencies but narrow bands of frequencies determined by the modulation thereon.
While the IF signals resulting from RF1 and RF2 have the same frequency, they may have different phase and carry different information. Thus, the IF signal may be thought of as having two components IF1, IF2 corresponding to two RF signals RF1, RF2, respectively. It is commonplace in the art to refer to one of the related signal pairs, e.g., RF1, IF1, as the "signal" and the other e.g., RF2, IF2, as the "image". The designations of RF1, IF1, as the "signal" and RF2, IF2 as the "image" are merely for convenience and may be interchanged. As used herein, the word "image" is intended to refer to IF1, IF2 collectively.
Mixers are often classified as "single balanced", double balanced", "image rejection" and "image separation", depending upon their configuration and whether or not they separate the RF images at the IF output. Of particular interest are image separation mixers, that is, mixers that provide separate outputs for IF1 and IF2.
A prior art image separation mixer is illustrated in FIG. 2. Mixer 10 has input 12 where, for example, RF input signal 14 comprising either or both RF1 and RF2 enters. Quadrature phase shifter 16 splits incoming signal 14 into two substantially equal amplitude RF signals 18, 20 which have a relative phase displacement of 90.degree. . Signal 18 is fed to three port mixer 19 and signal 20 is fed to three port mixer 21.
In FIG. 2 and subsequent figures, phase shifters are shown as a square with an inscribed diamond whose sides represent different paths between the ports of the phase shifter. The number adjacent each path identifies the relative phase shift along that path. For example, as signal 14 passes through the "0" path of phase shifter 16 to become signal 18 it undergoes a relative phase shift of 0.degree. and as signal 14 passes through the "90" path of shifter 16 to become signal 20 it undergoes a relative phase shift of +90.degree. with the result that signals 18 and 20 have a net or relative phase difference of 90.degree. . Those of skill in the art will understand that the phase shift amounts indicated on the phase shift symbols are relative phase shift amounts and not absolute phase shift amounts. This same convention is used in connection with FIGS. 2-5. Where a four-port shifter is used, it is conventional to couple the "isolation" port, e.g., port 17, to ground via resistor 23 as shown in FIG. 2.
Mixer 10 has input port 22 where, for example, LO signal 24 is provided. Power splitter 26 receives signal 24 and produces two substantially equal amplitude zero relative phase difference signals 28, 30 respectively. In-phase LO signals 28, 30 are fed to mixers 19, 21 where they are mixed with RF signals 18, 20, respectively to produce IF signals 32, 33 at their outputs. Input signals 14 and 24 may be interchanged, that is, the RF signal may be introduced at input 22 and the LO signal may be introduced at input 12. The operation of the circuit is substantially the same.
Intermediate frequency signals 32, 33 from mixers 19, 21 are applied to quadrature phase shifter 34 where they combine in such a way that RF signals RF1, RF2 at input 12 (or 22) produce IF signals IF1, IF2 separated at outputs 36, 38 respectively. Separation of the RF1, RF2 signals into IF1, IF2 signals occurs because the relative phase .phi. of the signals is different according to the relations: EQU .phi..sub.IF1 =.phi..sub.LO -.phi..sub.RF1, (4) EQU .phi..sub.IF2 =.phi..sub.RF2 -.phi..sub.LO, (4)
where .phi..sub.RF is the relative phase of the received signal at the RF frequency that may correspond to RF1 or RF2, .phi..sub.LO is the relative phase of the LO frequency, .phi..sub.IF1 is the relative phase of the output at the IF frequency produced by signal RF1 below LO, and .phi..sub.IF2 is the relative phase of the output at the IF frequency produced by signal RF2 above LO. Because of the phase relations of the intermediate IF signals 32, 33, at the IF1 output 36, the IF2 signal components are 180 degrees out of phase and cancel, while the IF1 signal components are in phase and thus combine or add. Likewise, at the IF2 output 38, the IF1 signal components are 180 degrees out of phase and cancel, while the IF2 signal components are in phase and thus combine or add. Thus this method of image separation and rejection is achieved by phase cancellations and additions, where the amount of image rejection is determined by the relative difference in IF1 and IF2 signal levels at either output port 36, 38. Equations (4)-(5) and the identification of the relative phase of the various signals are explained, for example, in B. J. Hallford, "Trace Phase States to Check Mixer Designs", Microwaves, Jun. 1980, pages 52-60.
FIG. 3 illustrates typical prior art three port mixer 40 used to provide mixers 19, 21 (FIG. 2). Mixer 40 has LO (or RF) input port 42 feeding into 180.degree. phase shifter 44 with isolation port 45 used for IF extraction and RF (or LO) input port 46 feeding into 180.degree. phase shifter 48 with isolation port resistive termination 47. The outputs from phase shifters 44, 48 are coupled to opposing ports of diode mixer ring 51. Mixers 19, 21 in prior art image separation mixer 10 (FIG. 2) each correspond to mixer 40 (FIG. 3).
Prior art image separation mixers have a number of disadvantages well known in the art. Among these are, for example, their relative complexity and the difficulty often encountered in implementation in compact form suitable for incorporation in monolithic microwave integrated circuits (MMIC's).
MMIC's are typically implemented using silicon, GaAs or other III-V integrated circuit (IC) wafer processing technology on and/or in such wafers. Thus, it is highly desirable to have image separation mixers which can be made with lumped elements or other structures that are compatible with IC fabrication techniques and geometry. In particular, it is important that they be of comparatively small in size so as to not occupy disproportionately large substrate areas compared to the semiconductor diodes, transistors, etc., which mix the signals, or compared to the amplifiers or other signal processing elements that may be included in the MMIC. Such concerns are especially important in the frequency range from about 1 to 15 GigaHertz where the size of distributed circuit elements is unwieldy.
Other disadvantages well know in the art are high conversion loss, poor isolation between the various ports and a high LO signal level requirement. Because the prior art image separation mixers typically use diodes as the non-linear elements, these mixers exhibit conversion loss in that the separated IF signals are smaller in amplitude than the received RF signals. Further, the use of diodes as the non-linear elements require a relatively large LO signal levels to drive the diodes in their normal operation. Another inherent limitation of prior art image rejection mixers is the limited signal isolation between the RF, LO and IF ports. Prior art schemes rely primarily on single phasing to achieve isolation.
Thus, there continues to be a need for improved image separation mixers which employ elements that result in a conversion gain rather than a conversion loss, which employ elements which permit a lower input LO signal level and improve the amount of isolation between the ports, and which at the same time can be implemented in MMIC form.